When a mobile device is used indoors, sound emitted by the user not only reaches the microphone of the mobile device directly, but also reaches the microphone after reflecting off objects such as the surrounding walls and ceiling. In the following description, sound that reaches a microphone directly will be designated direct sound, while sound that reaches the microphone after reflecting off objects such as the surrounding walls and ceiling will be designated reverb. Also, a signal obtained by the microphone in response to the arrival of sound will be designated an input signal.
For example, in a comparatively small room such as a bathroom, reverb reflected off the surroundings is greater compared to another place such as a living room. For this reason, when the telephony functions of a mobile device are used in a room such as bathroom, it may be difficult in some cases to generate clear sound from the input signal obtained by the microphone because of the superposition of direct sound and reverb.
Japanese Laid-open Patent Publication No. 2008-58900 proposes a technology that suppresses reverb components included in an input signal obtained by a microphone, in which a reverb power spectrum estimated from the power spectra of past frames is subtracted from the power spectrum of the current frame. This technique attempts reverberation suppression by determining filter coefficients so as to minimize a weighted sum of the residual speech power in a reverb segment at the end of an utterance and the subtracted power in an utterance segment, which are estimated on the basis of change in the input signal over time.